Satellite navigation using long-term navigation information

ABSTRACT

An improved approach to satellite-based navigation (e.g., GPS) is provided. In one embodiment, a method includes receiving a first set of tracking information. A nominal orbital path for the navigation satellite is determined using the first set of tracking information. Ephemeris data corresponding to the nominal orbital path is computed and uploaded to the navigation satellite. Long-term navigation information corresponding to the nominal orbital path is transmitted to a communication system for broadcast to a plurality of navigation devices. A second set of tracking information is received, an orbital path of the navigation satellite using the second set of tracking information is predicted, and a difference between the predicted orbital path and the nominal orbital path is determined. Commands configured to instruct the navigation satellite to adjust an actual orbital path of the navigation satellite to substantially conform to the nominal orbital path are uploaded to the navigation satellite.

TECHNICAL FIELD

The present invention relates generally to satellite-based navigation.

BACKGROUND

As is well known, conventional navigation devices compatible with theGlobal Positioning System (GPS) and Global Navigation Satellite System(GNSS) can aid users of such devices in determining their positionsrelative to various navigation satellites configured in a constellation.In this regard, satellite trajectory information is generally broadcastto such conventional navigation devices (e.g., GPS receivers), bysatellites as ephemeris data from which satellite positions can bepredicted. These devices then use the satellite ephemeris positioninformation along with ranging measurements to solve for their ownpositions using a triangulation process or similar process.

However, due to gravity perturbation effects (e.g., caused by the Earthgravity harmonics, sun, moon, and other bodies) and other externalperturbations (e.g., solar pressure disturbances), such ephemeris datais generally accurate for only short periods of time. Indeed, predictedsatellite positions determined using such ephemeris data may deviatefrom the true (i.e., actual) satellite positions by as much as a fewhundred meters for just a few hours outside of the time interval ofvalidity. Errors of this magnitude are too large for most GNSS basednavigation applications, and the trend over the past two decades hasbeen to reduce the Signal-in-Space (SIS) ranging error due to ephemerisand satellite clock from several meters to the current level of one ortwo meters for GPS satellites. The objective for future GNSS satellitesis to further reduce the ranging error to less than one meter.

As a result, satellites may be frequently uploaded (e.g., about once perday) with new ephemeris data sets to be broadcast by the satellites toconventional navigation devices. The nominally proposed solution toreduce errors for future GNSS is to upload new ephemeris data even morefrequently, e.g. once per hour, or once every few hours. Each piecewiseephemeris data set typically covers a limited time interval (e.g., about4 hours) which accounts for known and predictable forces (i.e., forcesother than variable solar pressure or other unpredictable small forces),with successive ephemeris data sets that overlap by one or two hours.

Unfortunately, there are significant disadvantages to this approach. Forexample, due to limited broadcast data rates, each navigation satelliteonly broadcasts the “current” ephemeris data set, and then switches to anew “current” ephemeris data set broadcast about every two hours.Conventional navigation devices must then typically read the new“current” ephemeris data set about every two hours to maintain fullaccuracy. In particular, if the conventional navigation devices rely onstale ephemeris data sets, accumulated deviations between the actualsatellite positions and the stale ephemeris data can result insignificant navigation errors.

In addition, the above approach renders conventional navigation devicessusceptible to navigation errors induced by the interruption ofsatellite ephemeris data broadcasts. Such interruptions may be caused,for example, by signal jamming, signal attenuation, line-of-sightblockage (e.g. urban canyon environments), weak Signal-to-Noise Ratio(SNR) conditions, or other forms of interference. While the navigationdevice can continue to make ranging measurements at very low signalpower levels or with a short span of data of one second or less, strongsignals are required to read new ephemeris information over the entiredata broadcast interval of up to 30 seconds. In this regard, thenavigation device may be configured to integrate the received signal forlonger intervals of time to filter noise effects while making rangingmeasurements (e.g., one second), whereas the integration interval fordemodulation of the broadcast ephemeris data is typically limited to 20milliseconds due to the broadcast data rate of 50 Hz. The ratio of thesetwo intervals (i.e., 50 or 17 dB), is an estimate of the relative weaksignal capability advantage associated with making ranging measurementsversus demodulating broadcast data.

In addition, once a navigation device is first turned on, it normallymust read new ephemeris data from the satellites. The time delay toobtain a solution after the device is turned on is known asTime-to-First Fix (TTFF). The delay of about 30 seconds just to readthis data often causes the TTFF to approach one minute for moststand-alone devices. Many users would prefer smaller TTFF of only a fewseconds.

As a result, there is a need for an improved approach to satellite-basednavigation that does not rely on frequent ephemeris updates to bereceived by conventional navigation devices. In particular, there is aneed for a satellite-based navigation approach that may permit userdevices to continue providing accurate position information for longintervals of time to improve TTFF and operational performance despitethe possible presence of satellite signal interference, line-of-sightobstructions, signal attenuation, or weak signal conditions.

SUMMARY

In accordance with one embodiment of the present invention, a method ofadjusting a navigation satellite orbit includes receiving a first set oftracking information for a navigation satellite; determining a nominalorbital path for the navigation satellite using the first set oftracking information; computing ephemeris data corresponding to thenominal orbital path; uploading the ephemeris data to the navigationsatellite; transmitting long-term navigation information correspondingto the nominal orbital path to a communication system for broadcast to aplurality of navigation devices; receiving a second set of trackinginformation for the navigation satellite; predicting an orbital path ofthe navigation satellite using the second set of tracking information;determining a difference between the predicted orbital path and thenominal orbital path; and uploading commands to the navigationsatellite, wherein the commands are configured to instruct thenavigation satellite to adjust an actual orbital path of the navigationsatellite to substantially conform to the nominal orbital path.

In accordance with another embodiment of the present invention, a methodof providing satellite-based navigation at a navigation device includesreceiving long-term navigation information from a communication system,wherein the long-term navigation information is associated with anominal orbital path of a navigation satellite; estimating a position ofthe navigation satellite using the long-term navigation information;repeating the receiving and the estimating operations for a plurality ofadditional navigation satellites using long-term navigation informationassociated with each of the additional navigation satellites; anddetermining a position of a navigation device based on the estimatedpositions of the navigation satellite and the additional navigationsatellites.

In accordance with another embodiment of the present invention, anavigation satellite includes a receive antenna adapted to receive aplurality of ephemeris data sets and commands, wherein the commands areconfigured to instruct the navigation satellite to adjust an actualorbital path of the navigation satellite to substantially conform to anominal orbital path; a memory adapted to store the ephemeris data sets;a transmit antenna adapted to transmit the ephemeris data sets to aplurality of navigation devices; a plurality of propulsion actuators;and a processor adapted to control the propulsion actuators in responseto the commands.

In accordance with another embodiment of the present invention, anavigation device includes means for receiving long-term navigationinformation from a communication system, wherein the long-termnavigation information is associated with nominal orbital paths of aplurality of navigation satellites; and a processor adapted to: estimatepositions of the navigation satellites using the long-term navigationinformation, and determine a position of the navigation device based onthe estimated positions of the navigation satellites.

The scope of the invention is defined by the claims, which areincorporated into this section by reference. A more completeunderstanding of embodiments of the present invention will be affordedto those skilled in the art, as well as a realization of additionaladvantages thereof, by a consideration of the following detaileddescription of one or more embodiments. Reference will be made to theappended sheets of drawings that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a satellite-based navigation system in accordance with anembodiment of the present invention.

FIG. 2 shows an exemplary navigation satellite that may be subject toorbital control in accordance with an embodiment of the presentinvention.

FIG. 3 shows a comparison of a true orbital path, a predicted orbitalpath, and a nominal orbital path in accordance with an embodiment of thepresent invention.

FIG. 4 is a flowchart showing operation of a satellite-based navigationsystem in accordance with an embodiment of the present invention.

Embodiments of the present invention and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

In accordance with various embodiments of the present invention, orbitcontrol may be used to eliminate or reduce the need for frequentshort-term ephemeris data updates to be periodically provided tonavigation devices from navigation satellites. In this regard, thetrajectories of one or more navigation satellites may be controlled tofollow nominal orbital paths corresponding to long-term navigationinformation.

In various embodiments, the use of long-term navigation information mayenable accurate prediction of navigation satellite orbital paths bynavigation devices for extended periods of time (e.g., longer thanapproximately 24 hours in one embodiment, and possibly several months inanother embodiment where navigation satellite orbits are successfullycontrolled without interruption to follow predicted long-term nominaltrajectories). As a result, navigation devices, once initialized withthe long-term navigation information, may continue to provide relevantnavigation position information to users despite the presence of jammingor other forms of interference that may interfere with the reception ofsatellite signals at the navigation devices.

In one embodiment, such long-term navigation information may be in theform of multiple sets of ephemeris data provided to navigation devices.In another embodiment, the long-term navigation information may be inthe form of state vector data provided to the navigation devices.

Referring now to the drawings wherein the showings are for purposes ofillustrating embodiments of the present invention only, and not forpurposes of limiting the same, FIG. 1 shows a satellite-based navigationsystem 100 in accordance with an embodiment of the present invention. Asillustrated in FIG. 1, system 100 includes a ground station 110, anavigation satellite 120, a navigation device 130, a tracking station140, and a communication system 150. Although a single one of each ofthe components of system 100 is illustrated in FIG. 1, it will beappreciated that system 100 may include a plurality of each of theillustrated components.

Tracking station 140 may be configured to measure the orbital range ofsatellite 120 and provide tracking information 160 to ground station110. It will be appreciated that such tracking information 160 may beprovided directly from tracking station 140 to ground station 110 and/orthrough one or more intermediate networks (not shown).

Ground station 110 may be configured with one or more appropriateprocessors and associated memories to perform various calculations. Forexample, ground station may be configured to perform orbitalcalculations based on tracking information 160 to determine a nominalorbital path for navigation satellite 120. In this regard, groundstation 110 may compute long-term navigation information 180corresponding to the nominal orbital path. For example, such long-termnavigation information 180 may be in the form of ephemeris data 180A orstate vector data 180B.

In one embodiment, ground station 110 may be configured to select adesired set of predicted forces affecting the orbital motion ofnavigation satellite 120. Ground station 110 may be further configuredto integrate differential equations associated with the selectedpredicted forces, starting from an initial condition, to generate thenominal orbital path for navigation satellite 120. Subsequentcorrections to the actual orbital patch of navigation satellite 120 maybe made to adjust the actual orbital path to substantially compensatefor deviations from the nominal orbital path caused by unpredictedforces such as, for example, solar radiation pressure.

As illustrated, ground station 110 may be further configured to transmitlong-term navigation information 180 (as multiple sets of ephemeris data180A or state vector data 180B) to communication system 150 forbroadcast to a plurality of navigation devices 130. Ground station 110may be further configured to periodically upload one or more sets ofephemeris data 180A or state vector data to navigation satellite 120.Whereas communication system 150 may be implemented with sufficientbandwidth to quickly transmit multiple sets of long-term navigationinformation 180 for the entire constellation of navigation satellites120 to the navigation device 130, each navigation satellite 120, due tolimited broadcast bandwidth, typically only broadcasts its “current”ephemeris parameter data set.

Ground station 110 may also be configured to predict an orbital path ofnavigation satellite 120 using tracking information 160 and determinedifferences between the predicted orbital path and nominal orbital path.In addition, ground station 110 may be configured to prepare and uploadcommands 190 to navigation satellite 120. Such commands 190 may beconfigured to instruct navigation satellite 120 to adjust its actualorbital path via thruster firings to substantially conform to thenominal orbital path.

Communication system 150 may be configured as any appropriate wired orwireless communication system or network which may be used to providelong-term navigation information 180 to navigation devices 130. Forexample, in one embodiment, communication system 150 may be implementedas a non-GNSS communication system. In another embodiment, communicationsystem 150 may be implemented by the Internet or a conventional wirelessnetwork. In various embodiments, the transmission and broadcast oflong-term navigation information 180 may be performed over atransmission channel other than that used by conventional (i.e., legacy)navigation devices and may optionally include secure methods ofcommunication and encryption.

Navigation device 130 may be configured to receive long-term navigationinformation 180 from communication system 150 and/or navigationsatellite 120. In this regard, navigation device 130 may include one ormore antennas (not shown) for receiving satellite signals. Such antennasmay also enable communication with communication system 150 and/orground station 110 for requesting and/or receiving wireless signalsincluding long-term navigation information 180. Navigation device mayalso include an appropriate data port to facilitate a direct wiredconnection with communication system 150 (e.g., to permit monthly orother periodic wired connections over the Internet using, for example, adocking cradle or cable).

Navigation device 130 may further include one or more appropriateprocessors and associated memories capable of processing receivedsignals and for executing instructions stored in a machine-readablemedium in order to determine the position, velocity, and time trajectoryof navigation device 130 using long-term navigation information 180.

Navigation satellite 120 can be any type of spacecraft that may be usedto receive long-term navigation information 180 and commands 190,transmit current data sets of long-term navigation information 180 toone or more navigation devices 130, and adjust its actual orbital pathin response to commands 190.

Ground station 110 may compute long-term navigation information 180corresponding to the nominal orbital path of navigation satellite 120 byaccounting for acceleration forces affecting the orbit of navigationsatellite 120. In this regard, the following Equation (1) may be used asa differential equation of motion to account for such accelerationforces to determine the nominal orbital path:

$\begin{matrix}{\frac{^{2}{\overset{arrow}{r}}_{cg}}{t^{2}} = {{\overset{¨}{\overset{arrow}{r}}}_{cg} = {{{- \frac{\mu}{r_{cg}^{3}}}( {\overset{arrow}{r}}_{cg} )} + {\overset{arrow}{a}}_{E} + {\overset{arrow}{a}}_{m} + {\overset{arrow}{a}}_{s} + {{\overset{arrow}{a}}_{sp}( {\overset{arrow}{\theta},A_{sp},\overset{arrow}{S}} )}}}} & (1)\end{matrix}$

The first term in Equation (1) represents the two-body dominantacceleration term due to Earth gravity, followed by the Earth harmonicsand tides terms, each of which are a function of latitude, longitude,and altitude. The third and fourth terms represent gravity forces of themoon and sun, respectively. The last term of Equation (1) is theradiation pressure as a function of the satellite attitude (θ),reflectivity and solar pressure model parameters (Asp, which depends onarea), the distance to the sun, radiant output of the sun, and sun linedirection (S).

Considering Equation (1), an ideal nominal trajectory of navigationsatellite 120 would only consist of the dominant first term. With thisselection, the ideal nominal orbital trajectory would consist of theclosed-form two-body solution given by Kepler's equation. However, withthis selection for the nominal trajectory, the propellant to controlcancellation of all of the remaining force perturbations could beexcessive.

In one embodiment, to keep propellant requirements small, generation ofthe nominal trajectory of the navigation satellite 120 by the groundstation 110 may be based on all of the well-known, predictable forces inEquation (1), including all of the gravity perturbation terms and, ifdesired, the nominal solar pressure force. The major unpredictable forcewould then be associated with the uncertainty or error in the solarpressure model. Thus, with this selection for generation of the nominaltrajectory, the thrust control action would only need to periodicallycorrect for the small uncertainty in solar pressure effects. In oneembodiment, the propellant consumption using high specific impulsethrusters only amounts to a few kilograms over 10 years.

There is no closed form solution for an ephemeris representation ofnavigation satellite 120 that remains accurate enough for most GNSSbased navigation applications for time periods greater than severalhours (i.e., due to secular variations in the orbital right ascension ofthe ascending node, perigee, and mean anomaly Keplerian parameterscaused by the gravity of the moon, sun, and Earth harmonics).Accordingly, ground station 110 may be configured to generate long-termnavigation information 180 that includes many sets of ephemeris data180A, each of which may be valid for several hours, and adjacent datasets corresponding to consecutive, but possibly overlapping timeintervals of the long-term nominal orbit trajectory. Advantageously, theeffects of any other unpredictable perturbations beyond solar pressure(e.g., higher order terms from Polar Wander, Earth rotation, and initialvelocity error) are also removed by daily uploaded commands 190 to bringthe satellite trajectory back into approximate agreement with thenominal trajectory. In one embodiment, such ephemeris data 180A may alsobe used by conventional GPS/GNSS satellites if desired. In this regard,the GPS Master Control Station (MCS) Kalman filter, which operates toestimate GPS satellite orbit trajectories and clock states, couldincorporate predicted small force effects of thrust commands into itsorbital determination computations, and daily ephemeris uploads to thesatellites would proceed as normally to continue to service conventionalGPS satellite navigation messages and navigation devices withoutdegradation.

In one embodiment, a limited number of ephemeris data 180A data sets(e.g., corresponding to ephemeris data used for approximately one day ofsuccessive ephemeris satellite broadcasts) may be uploaded to navigationsatellite 120 on a daily basis. In another embodiment, a large number ofephemeris data 180A data sets (e.g., corresponding to more than one dayof successive ephemeris satellite broadcasts) may be uploaded tonavigation satellite 120 and transmitted to communication system 150(for broadcast to navigation devices 130) at intervals greater thanapproximately 24 hours. However, whereas the high-bandwidthcommunication system 150 can transmit large number ephemeris data 180Adata sets within a short interval or time, each navigation satellite 120typically only broadcasts its “current” ephemeris data 180A data set dueto limited bandwidth and operational constraints.

In one embodiment, ephemeris data 180A may be represented using existingGPS navigation messages. In this case, the amount of data provided foreach successive ephemeris data 180A data set satellite broadcast may beonly about 1000 bits. Accordingly, a large number of ephemeris data 180Adata sets corresponding to ephemeris data used for approximately onemonth of successive ephemeris satellite broadcasts may be less than 1Mbit for each navigation satellite 120. In one embodiment, communicationsystem 150 may operate at a data rate of several megabits per second(Mbps). In such an embodiment, communication system 150 can quicklytransmit a large number of ephemeris data 180A data sets to navigationdevice 130 within a short period of time. On the other hand, wherenavigation satellite 120 broadcasts data at a rate of 50 bps, navigationsatellite 120 may only broadcast its “current” ephemeris data 180A dataset to navigation device 130.

As previously discussed, long-term navigation information 180 may alsobe provided in the form of state vector data 180B, including theposition, velocity, and solar pressure states of the satellite 120 at anepoch time. In this approach, long-term navigation information 180 mayinclude one or more initial state vectors, nominal Earth OrientationParameters, and nominal force models which may be used by navigationdevice 130 to compute nominal orbital path 330 in accordance withEquation (1) previously discussed above. Specifically, navigation device130 may be configured to numerically integrate Equation (1) using aninitial state vector and nominal force models to replicate the GPS MCScalculations for prediction of the nominal trajectory, and thus predictthe trajectory of navigation satellite 120 at future times. In anotherembodiment, state vector data 180B may also be transmitted to navigationsatellite 120 and subsequently broadcast by navigation satellite 120 touser device 130.

FIG. 2 shows an embodiment of navigation satellite 120 that may besubject to orbital control in accordance with the present invention. Inone embodiment, navigation satellite 120 may be implemented as a GPS orGNSS compatible satellite including a receive antenna adapted to receiveephemeris data 180A and/or state vector data 180B configured andformatted in accordance with the form of legacy GPS/GNSS ephemeris datauploads, as well as commands 190. In such an embodiment, navigationsatellite 120 may further include a memory adapted to store ephemerisdata 180A and/or state vector data 180A, and a transmit antenna adaptedto transmit ephemeris data 180A and/or state vector data 180A tonavigation device 130.

As shown, navigation satellite 120 includes one or more propulsionactuators (e.g., thrusters) 125 implemented, for example, as xenon ionpropulsion thrusters (as illustrated), electric propulsion actuators, orother appropriate propulsion actuators. Such propulsion systems tend tobe very low-thrust and very propellant efficient, and therefore arewell-suited for providing a continuous-time counterforce to offsetexternal small forces without causing either rotational or translationaldisturbances to navigation satellite 120.

Following is a calculation which may be used to estimate the amount ofpropellant used to cancel the effects of orbital disturbances induced bythe solar pressure on one embodiment of navigation satellite 120. Inthis case, the dominant disturbing external force is the solar radiationpressure forces. Accordingly, the solar radiation force equation may berepresented by the following Equation (2):

F _(solar)=−4.5×10⁻⁶(1+r)A   (2)

where A is the satellite cross-sectional area exposed to the sun in m2,and r is a material reflection factor (where r=0 for absorption; r=1 forspecular reflection at normal incidence).

In one embodiment, navigation satellite 120 may exhibit a total area ofapproximately 27.5 m2 that is exposed to the sun at any one time. Insuch an embodiment, the mass of navigation satellite 120 may beapproximately 1700 kg. Using Equation (2), the solar pressure force maybe roughly computed to be in the range of about 0.13 to about 0.25 milliNewton (mN) depending on the material properties of navigation satellite120.

A 13 cm xenon ion thruster delivers about 18 mN of thrust with a veryefficient thruster specific impulse (Isp) of 2600 sec. Such a thrusteror a smaller thruster (e.g., about 3 cm diameter smaller) used forpropulsion actuators 125 may be used to cancel the dominant solarpressure and other perturbation forces and provide satisfactory thrustwith extra margin in order to maintain a desired orbital path inaccordance with various embodiments of the invention discussed herein.

FIG. 3 shows a comparison of a true orbital path 310, a predictedorbital path 320, and a nominal orbital path 330 in accordance with anembodiment of the present invention. In this regard, true orbital path310 represents the actual orbital path of navigation satellite 120 inspace relative to the Earth. Predicted orbital path 320 represents theorbital path of navigation satellite 120 as measured by tracking station140 and predicted by the ground station 110. In this regard, it will beappreciated that predicted orbital path 320 may deviate from trueorbital path 310 due to measurement and prediction errors. Nominalorbital path 330 represents an orbital path conforming to long-termnavigation information 180 provided by ground station 110.

As discussed above, ground station 110 may receive tracking information160 from tracking station 140 and may compute long-term navigationinformation 180 in the form of ephemeris data 180A which is compatiblewith legacy GPS/GNSS systems. As shown in FIG. 3, ground station 110 mayupload sets of ephemeris data 180A to navigation satellite 120 thatinclude ephemeris data to be used every few hours in successiveephemeris satellite broadcasts. Navigation satellite 120 maycontinuously broadcast a current set of ephemeris data 180A and switchto broadcasting the next set of ephemeris data 180A every few hours(e.g., about every 2 hours).

As also previously discussed, ground station 110 may compute orbitalpath correction commands 190 to be uploaded to satellite 120 for controlof propulsion actuators 125. In this regard, ground station 110 mayupload commands 190 at intervals of, for example, approximately 24hours. In response to each command upload operation, navigationsatellite 120 may be configured to adjust its true orbital path 310 toalign more closely with nominal orbital path 330. For example, as shownin FIG. 3, true orbital path 310 and predicted orbital path 320 ofnavigation satellite 120 may change in response to a first commandupload operation (labeled 190A) which causes such orbital paths to moreclosely align with nominal orbital path 330. It will be appreciated thatthe operations illustrated in FIG. 3 may be repeated for all navigationsatellites 120 in a given satellite constellation.

FIG. 4 is a flowchart showing operation of satellite-based navigationsystem 100 in accordance with an embodiment of the present invention. Asillustrated, the process of FIG. 4 includes four major segments: controlsegment 401, which corresponds substantially to operations performed byground station 110, tracking station 140, and/or communication system150; space segment 402, which corresponds substantially to operationsperformed by navigation satellite 120; new user segment 403A, whichcorresponds substantially to operations performed by navigation device130 when using long-term navigation information 180 received fromcommunication system 150; and legacy user segment 403B, whichcorresponds substantially to operations performed by navigation device130 when using ephemeris data 180A received from navigation satellite120.

Turning now to the particular operations of FIG. 4, in block 405,tracking station 140 measures predicted orbital path 320 of navigationsatellite 120 and provides a corresponding initial set of trackinginformation 160 to ground station 110. Then, in block 407, groundstation 110 determines nominal orbital path 330 of navigation satellite120 and computes long-term navigation information 180 in the form ofephemeris data 180A for nominal orbital path 330.

As shown in FIG. 4, the computations performed in block 407 may be usedby subsequent blocks 410, 435, and 450. Turning now to block 410, groundstation 110 uploads ephemeris data 180A to navigation satellite 120. Inone embodiment, the upload operation of block 410 may be implemented asa daily upload of sets of ephemeris data 180A (data sets in this dailyupload may also be referred to as short duration ephemeris data)corresponding to approximately one day of satellite broadcasts.

In block 415, a plurality of navigation satellites 120 periodicallybroadcast their current associated ephemeris data 180A to navigationdevice 130. In block 420, navigation device 130 estimates and predictsthe position of the plurality of navigation satellites 120 using thebroadcasted ephemeris data 180A in accordance with legacy GPS/GNSStechniques. Then, in block 425, navigation device 130 determines its ownposition based on the positions of the plurality of navigationsatellites 120 determined in block 420. Accordingly, following block425, navigation device 130 will have obtained its position usinglegacy-based GPS/GNSS navigation techniques.

Turning now to block 450, ground station 140 transmits long-termnavigation information 180 in the form of ephemeris data 180A tocommunication system 150. In one embodiment, the transmit operation ofblock 450 may include a large number of sets of ephemeris data 180A(this large number of data sets may also be referred to as long durationephemeris data) corresponding to a portion of nominal orbital path 330spanning greater than one day, and possibly several months. In block455, communication system 150 broadcasts the long duration ephemerisdata to navigation device 130 through, for example, a wireless network,the Internet, wired connection, or another desired non-GNSS basedcommunication channel (block 455).

In one embodiment, navigation device 130 may be selectively authorizedto receive long-term navigation information 180 from communicationsystem 150. In this regard, it is contemplated that navigation device130 may subscribe, for example, to a fee-based service which mayauthorize navigation device 130 to receive long-term navigationinformation 180.

In block 460, navigation device 130 may estimate and predict theposition of the plurality of navigation satellites 120 using ephemerisdata 180A received either from the broadcast by navigation satellites120 in block 415 or from the broadcast by communication system 150 inblock 455. In this regard, it will be appreciated that navigation device130 may select whether to use ephemeris data 180A received fromnavigation satellite 120 or from communication system 150, depending onwhether communications from navigation satellite or communication system150 are currently available. For example, if navigation device 130 isunable to receive ephemeris data 180A from navigation satellite 120 dueto jamming, interference, or other conditions, navigation device 130 maystill be able to receive ephemeris data 180A from communication system150.

Navigation device 130 may also choose to use ephemeris data 180Areceived from communication system 150 to reduce TTFF. Because thetransmit operation of block 450 may include ephemeris data 180Acorresponding to a portion of nominal orbital path 330 spanning greaterthan one day and possibly several months, navigation device 130 maycontinue to use such data for an extended period of time (e.g., longerthan 24 hours, and possibly several months) if satellite signals remaininterrupted.

In block 465, navigation device 130 determines its own position based onthe positions of the plurality of navigation satellites 120 determinedin block 460. Accordingly, following block 425, navigation device 130will have obtained its position using either legacy-based GPS/GNSSnavigation techniques or long-term navigation information 180 broadcastby communication system 150. In one embodiment, navigation device 130may be configured to check the consistency of long duration ephemerisdata available from communication system 150 and short durationephemeris data available from navigation satellite 120.

Turning now to block 430, tracking station 140 measures predictedorbital path 320 of navigation satellite 120 and provides acorresponding subsequent set of tracking information 160 to groundstation 110. Then, in block 432, ground station 110 estimates predictedorbital path 320 of navigation satellite 120 based on the subsequent setof tracking information 160. It will be appreciated that block 430 maybe performed at a later time than block 405 previously discussed above.Accordingly, because navigation satellite 120 is in motion, thesubsequent set of tracking information 160 provided in block 430 maydiffer from the initial set of tracking information 160 provided inblock 405. As a result, in block 435, ground station 110 determines adifference between predicted orbital path 320 (which was provided inblock 432) and nominal orbital path 330 (which was provided in block407).

In block 440, ground station 110 prepares commands 190 that areconfigured to instruct navigation satellite 120 to adjust its trueorbital path 310 to substantially conform to nominal orbital path 330,and uploads commands 190 to navigation satellite 120. In response,navigation satellite 120 executes commands 190 using one or moreappropriate processors and propulsion actuators to adjust its trueorbital path 310 accordingly. Following block 445, the process of FIG. 4may loop back to block 430 in which additional sets of trackinginformation 160 may be provided to ground stations 110, and the trueorbital path 310 of navigation satellite 120 may be iteratively adjustedto substantially conform to nominal orbital path 330.

It will be appreciated that following block 445; true orbital path 310of navigation satellite 120 may be moved to substantially conform tonominal orbital path 330. As a result, ephemeris data 180A previouslycalculated in block 407 and broadcast to navigation devices 130 inblocks 415 and 455, respectively, may continue to be an accuraterepresentation of the position of navigation satellite 120. It willfurther be appreciated that, where appropriate, the various operationsdiscussed in the blocks of FIG. 4 may be performed for all navigationsatellites 120 in a given satellite constellation.

In another embodiment, the process of FIG. 4 may be modified to usestate vector data 180B. In this regard, ground station 110 may alsocompute state vector data 180B for nominal orbital path 330 in block407, and then upload and transmit state vector data 180B in place ofephemeris data 180A in blocks 410 and 450, respectively. Similarly,navigation satellite 120 and communication system 150 may broadcaststate vector data 180B (which may also include navigation satellite 120solar pressure nominal force model data) in place of ephemeris data 180Ain blocks 415 and 455. In one embodiment, state vector data 180Buploaded to, and broadcast from navigation satellite 120 may be“current” state vector data 180B of limited duration.

Also in this embodiment, in block 460, navigation device 130 mayestimate and predict the position of the plurality of navigationsatellites 120 using either ephemeris data 180A or state vector data180B. If state vector data 180B is used, then navigation device 130 maydetermine the positions of the plurality of navigation satellites 120using state vector data 180B instead of ephemeris data 180A. Then, inblock 465, navigation device 130 may determine its own position based onthe positions of the plurality of navigation satellites 120 determinedin block 460.

It will be appreciated that an improved approach to satellite-basednavigation in accordance with various embodiments disclosed herein canaccurately predict the trajectories and orbital paths of navigationsatellites for long periods of time (for example, several months ormore), without requiring navigation devices to rely on ephemeris databroadcast directly from such navigation satellites.

Advantageously, navigation devices configured in accordance with suchembodiments may obtain long-term navigation information prior to startof a navigation-dependent mission, and may continue to use suchlong-term navigation information in the event that navigation satellitetransmissions are interrupted or corrupted by signal jamming or weakenedby interference due to line-of-sight obstruction (e.g., in urban canyonenvironments) or multi-path problems.

Additionally, because such navigation devices may already be inpossession of the long-term navigation information, such devices mayachieve a faster TTFF lock on satellite navigation signals after turn onthan legacy-based navigation devices. In this condition, as soon as timehas been determined, a navigation device can compute a position fix. Forexample, a navigation device in accordance with one embodiment of theinvention may provide a TTFF of only a few seconds while, in contrast, alegacy-based navigation device may provide a TTFF of about 40 seconds orso since 30 seconds are required to read new ephemeris data as broadcastfrom navigation satellites.

A navigation device in accordance with one embodiment of the inventionmay advantageously be implemented such that access time to long-termnavigation information stored in onboard memory may be significantlyshorter than acquisition of data broadcast from navigation satellites.The access frequency of the ephemeris data may also be substantiallyimproved (i.e., ephemeris data may be accessed about once every twohours from navigation satellite 120, whereas an access of once per weekor once per month may be sufficient from communication system 150). Thiscontrasts with conventional devices which may need to wait until a newdata set page is broadcast every two hours (e.g., in 20-30 second repeatintervals), or when a new satellite appears above the horizon.

Civilian users may benefit from using embodiments of the presentinvention in urban canyon and indoor environments, when legacy signalattenuation, line-of-sight obstructions, and multi-path effects maycompromise or corrupt the legacy GPS broadcast data. Therefore,navigation devices may obtain faster TTFF in these environments.Furthermore, wireless service providers (e.g. cell phone operators) mayexperience reduced data rate loads in associated GPS wireless assistnetworks due to less frequent ephemeris data updates.

Advantageously, in one embodiment, conventional navigation devices maycontinue to receive long-term navigation information as ephemeris datafrom navigation satellites in compatible legacy GPS/GNSS formats. Inanother embodiment, such conventional navigation devices may be modifiedto accommodate the storage of long duration ephemeris data sets and uselong duration sets of ephemeris data rather than relying on legacysatellite in weak or corrupted signal environments.

Satellite operators may also benefit from various embodiments of thepresent invention in which operators may precisely predict thenavigation satellite trajectories for long durations (e.g., severalweeks or months). Satellite operators may also benefit due to areduction or elimination of system down-time due to orbit maneuvers(i.e., station keeping) since navigation satellites in accordance withvarious embodiments disclosed herein may be configured to performcontinuous fine orbit control to minimize errors between predicted andnominal orbital paths. In one embodiment, software of conventionalnavigation satellites may be advantageously modified to accommodatecommands to facilitate orbit control using appropriate propulsionactuators.

Embodiments described above illustrate but do not limit the invention.It should also be understood that numerous modifications and variationsare possible in accordance with the principles of the present invention.Accordingly, the scope of the invention is defined only by the followingclaims.

1. A method of adjusting a navigation satellite orbit, the methodcomprising: receiving a first set of tracking information for anavigation satellite; determining a nominal orbital path for thenavigation satellite using the first set of tracking information;computing ephemeris data corresponding to the nominal orbital path;uploading the ephemeris data to the navigation satellite; transmittinglong-term navigation information corresponding to the nominal orbitalpath to a communication system for broadcast to a plurality ofnavigation devices; receiving a second set of tracking information forthe navigation satellite; predicting an orbital path of the navigationsatellite using the second set of tracking information; determining adifference between the predicted orbital path and the nominal orbitalpath; and uploading commands to the navigation satellite, wherein thecommands are configured to instruct the navigation satellite to adjustan actual orbital path of the navigation satellite to substantiallyconform to the nominal orbital path.
 2. The method of claim 1, whereinthe determining comprises: selecting a set of predicted forces affectingorbital motion of the navigation satellite; and numerically integratingdifferential equations of motion corresponding to the predicted forcesstarting from an initial condition to generate the nominal orbital pathfor the navigation satellite.
 3. The method of claim 2, wherein thecommands are configured to instruct the navigation satellite to adjustthe actual orbital path to substantially compensate for deviations fromthe nominal orbital path caused by unpredicted forces.
 4. The method ofclaim 2, wherein the selecting is performed to control propellantconsumption by the navigation satellite.
 5. The method of claim 2,wherein the selecting is performed to reduce periodic navigation stationkeeping maneuvers.
 6. The method of claim 1, wherein the long-termnavigation information is the ephemeris data.
 7. The method of claim 1,wherein the long-term navigation information is state vector data. 8.The method of claim 7, wherein the state vector data comprises models ofearth gravity, earth harmonics, earth tides, moon gravity, sun gravity,solar radiation pressure, and predicted satellite trajectory effectsbased on application of translational control forces.
 9. The method ofclaim 1, further comprising repeating the method for a plurality ofadditional navigation satellites.
 10. The method of claim 1, furthercomprising repeating the uploading at intervals of approximately 24hours.
 11. The method of claim 1, further comprising repeating thetransmitting at intervals greater than approximately 24 hours.
 12. Themethod of claim 1, wherein the communication system is the Internet. 13.The method of claim 1, wherein the communication system is a wirelesscommunication system.
 14. A method of providing satellite-basednavigation at a navigation device, the method comprising: receivinglong-term navigation information from a communication system, whereinthe long-term navigation information is associated with a nominalorbital path of a navigation satellite; estimating a position of thenavigation satellite using the long-term navigation information;repeating the receiving and the estimating operations for a plurality ofadditional navigation satellites using long-term navigation informationassociated with each of the additional navigation satellites; anddetermining a position of the navigation device based on the estimatedpositions of the navigation satellite and the additional navigationsatellites.
 15. The method of claim 14, wherein the long-term navigationinformation is ephemeris data corresponding to the nominal orbital path.16. The method of claim 14, wherein the long-term navigation informationis state vector data.
 17. The method of claim 16, wherein the statevector data comprises models of earth gravity, earth harmonics, earthtides, moon gravity, sun gravity, solar radiation pressure, andpredicted satellite trajectory effects based on application oftranslational control forces.
 18. The method of claim 14, furthercomprising: receiving ephemeris data from the navigation satellite,wherein the ephemeris data corresponds to the nominal orbital path ofthe navigation satellite; selecting the long-term navigation informationreceived from the communication system or the ephemeris data receivedfrom the navigation satellite; and estimating the position of thenavigation satellite based on the selected long-term navigationinformation or the selected ephemeris data.
 19. The method of claim 18,further comprising: repeating the second receiving, the selecting, andthe second estimating operations for the plurality of navigationsatellites and long-term navigation information or ephemeris dataassociated with the plurality of navigation satellites; and determiningthe position of the navigation device based on the estimated positionsof the navigation satellites.
 20. The method of claim 14, wherein thecommunication system is the Internet.
 21. The method of claim 14,wherein the communication system is a wireless communication system. 22.The method of claim 14, wherein the method is performed by thenavigation device pursuant to a fee-based service agreement.
 23. Anavigation satellite comprising: a receive antenna adapted to receive aplurality of ephemeris data sets and commands, wherein the commands areconfigured instruct the navigation satellite to adjust an actual orbitalpath of the navigation satellite to substantially conform to a nominalorbital path; a memory adapted to store the ephemeris data sets; atransmit antenna adapted to transmit the ephemeris data sets to aplurality of navigation devices; a plurality of propulsion actuators;and a processor adapted to control the propulsion actuators in responseto the commands.
 24. The navigation satellite of claim 23, wherein thepropulsion actuators are selected from the group consisting of: ionpropulsion actuators and electric propulsion actuators.
 25. A navigationdevice comprising: means for receiving long-term navigation informationfrom a communication system, wherein the long-term navigationinformation is associated with nominal orbital paths of a plurality ofnavigation satellites; and a processor adapted to: estimate positions ofthe navigation satellites using the long-term navigation information,and determine a position of the navigation device based on the estimatedpositions of the navigation satellites.
 26. The navigation device ofclaim 25, further comprising: means for receiving ephemeris data fromthe navigation satellites, wherein the ephemeris data corresponds tonominal orbital paths of the navigation satellites, wherein theprocessor is further adapted to: compare the long-term navigationinformation received from the communication system to the ephemeris datareceived from the navigation satellites as a consistency check, selectthe long-term navigation information received from the communicationsystem or the ephemeris data received from the navigation satellites,and estimate the positions of the navigation satellites based on theselected long-term navigation information or the selected ephemerisdata.
 27. The navigation device of claim 25, wherein the long-termnavigation information is ephemeris data corresponding to the nominalorbital paths of the navigation satellites.
 28. The navigation device ofclaim 25, wherein the long-term navigation information is state vectordata.
 29. The navigation device of claim 25, wherein the state vectordata comprises models of earth gravity, earth harmonics, earth tides,moon gravity, sun gravity, solar radiation pressure, and predictedsatellite trajectory effects based on application of translationalcontrol forces.